Frogs, Puffer Fish and Poison
by Goldenpupil
Summary: One shot. AU ending to ASIP. If John did not see the standoff between Sherlock and the cabbie from the other building, and fired his gun, would Sherlock have outsmarted the cabbie?


**Frogs, Puffer Fish, and Poison**

One shot. AU ending to ASIP. If John did not see the standoff between Sherlock and the cabbie from the other building, and fired his gun, would Sherlock have outsmarted the cabbie?

Disclaimer: Based on characters of BBC Sherlock. No copyright infringements intended; no personal wealth gained. Might have also used ideas from other fictions – don't remember any names – very sincere apologies.

* * *

><p>The short, graying cabbie stood facing the tall, dark haired detective, both men in nearly identical postures. Each had one hand clenching a clear bottle, and the other holding a single white, gold specked capsule.<p>

The cabbie wore a cynical, almost maniac leer as he goaded the detective, but his face was pale and damp with sweat, and the stubby hand clutching the bottle trembled, just a little. Whether he was overeager to announce his victory or in fear of his imminent death, it was impossible to tell.

Sherlock himself was a shade paler than usual, but his hand was steady as he lifted the capsule to the light and trained his steel grey eyes on the small white object, his aquiline profile focused and exhilarated: a worthy puzzle to unravel. And he had never been wrong, so far.

The air was thick with tension, adrenaline, and thrill. This was a game, the ultimate gamble, and both men wanted to be right. Needed to be right.

One of them already knew exactly how it was going to end.

The cabbie's reedy, accented voice echoed in the empty classroom, quiet, lightly slurred, taunting.

Sherlock lowered the capsule, his lips parting.

* * *

><p>John burst though the wooden door, revealing yet another empty classroom.<p>

Boarded windows. Tables and chairs. No Sherlock.

If John felt the choking, sickening sense of fear rise in his throat, he didn't show it. With the efficiency and focus of a military man, John moved on to the next door.

* * *

><p>Sherlock's piercing gaze, previously drilling into the capsule in front of his lips, snapped up suddenly to the cabbie. He lowered the capsule and spoke.<p>

"Give me a name." Sherlock's keen eyes glinted at the cabbie's startled expression at the non sequitur. "Who directed you in this? Your murders are clean, reasonably well planned – only four major mistakes – taking your last victim's suitcase was one. I'll say it again: you're wastedas a cabbie. But _this _–" Sherlock's eyes narrowed, "the choice between two bottles, the poisoned pill – it's a bit too clever, even for you. You didn't just get money for serial killing; you also got a few tips from your 'sponsor', didn't you? Give me the name."

The cabbie was silent; impatience crept into Sherlock's voice. "Come on now … aren't you confident that I will die? So what is it to you if I know the name?"

The cabbie's voice was flat. "They know where my kids are. Had to tell 'em where to send the money, didn't I? I was told not to ever say the name, and I'm not stupid enough to risk it."

"But I thought you liked risks – weren't those your own words, _cabbie_?" Sherlock's low voice had become soft, as taunting as the cabbie's had been. "Satisfying a dead man's curiosity won't hurt you."

"Won't help me either, will it?"

Sherlock immediately took a step forward, voice lowering to an eager whisper, "I'll play your little game in exchange for the name. Tell me now, or I'll walk away." A small smirk. "You want to play this game as much as I do … you're addicted to the killing – will you really just let me, a victim in your power, leave?"

The silence stretched on. Sherlock didn't break it.

"Moriarty." The cabbie spoke quietly, then smirked as Sherlock immediately tilted his head back, mouthing the word with relish.

Then the cabbie made a show of checking his wrist watch. Sherlock's eyes flickered towards it.

"You've been stalling, Mr. Holmes." The cabbie's smirk became uglier, and he drew a small knife from his pocket, flicking the blade out. Sherlock glanced at it condescendingly.

"You have your name, now you've got to play. Did you think I would kidnap someone without any real weapons on me? Now, you really don't want to be fighting a man who's gonna die anyway, Mr. Holmes. You won't win." The maniac smile was back on the cabbie's face; his whole torso was now quivering with what seemed to be perverse excitement. "So take your medicine, Sherlock."

Sherlock cocked his head. "No."

"What'd you say?"

"No, I won't take it. It's poisoned."

The cabbie laughed, throwing his head back. "Having second thoughts, Mr. Holmes? Now, I don't usually allow take backs … but maybe I'll bend the rules and let you switch with me … just this once."

"Why would I want yours?" retorted Sherlock, unfazed, "Yours is also poisoned."

For a moment, the cabbie blinked, surprised, then shook with laughter again, lips twisted in patronizing amusement, like he was indulging a child. "Ooh, he thinks I'm cheating, doesn't he? But one tiny problem with your theory, Mr. Holmes … I've played this game before, remember? How did I take a poisoned pill and live?"

"Simple. Because you are immune."

The cabbie stopped laughing, and looked at Sherlock as if pitying him for making the ludicrous statement. "No one's immune to _this_ poison, Mr. Holmes."

"You can be." Sherlock raised the capsule to the light once more, "This is Batrachotoxin – an alkaloid extract of poison dart frog skin secretion – I can see the small, white crystals that are mixed with the inert lactose padding in the capsule … That's another mistake you made, by the way, using an easily recognizable poison: Steroidal alkaloid crystals all have a distinctive shape, not to mention a noticeable smell that was present in the breath of your victims, and among alkaloid poisons only Batrachotoxin could have caused nervous contractions fast enough and severe enough to result in your latest victim vomiting without control until she asphyxiated … but it would have to be a weaker Batrachotoxin or you wouldn't even be able to make these pills without poisoning yourself –"

"Very nice, Mr. Holmes," The cabbie interrupted, breath harsh, all humor lost, "So you worked out the poison. Like I said, proper thinking. So what?"

"Don't pretend you don't know – so _everything_!" Sherlock backed a few steps, eyes wide and excited, spreading out his arms, and began talking at bullet speed: "All Batrachotoxins disrupt the sodium ion flux in the nervous system – instantly fatal, but the effects of the weaker varieties can be counteracted by a strong enough substance with the opposite effects on the sodium flux, but it's no use to take it _after_ you've taken the poison, you don't absorb anything else as fast as you absorb steroidal alkaloids … no, you have to take it _before_!" Sherlock's eyes glittered as they swept over the now silent cabbie, gaze lingering on the cabbie's pale, sweaty face and twitching hands.

"You take the antidote prior to challenging your victims, and then you take the Batrachotoxin with the antidote still in your system, canceling its effects…'_You know how people think…'_" Sherlock sneered derisively, "Oh please, all you can do is cheat at your own little game: immunize yourself to the poison, and then let your victim choose from two poisoned pills, so you always 'win'."

The cabbie's jaw seemed to have frozen. His chest was heaving, and his hands, one clutching the bottle and the other the knife, were shaking increasingly violently.

Sherlock continued ruthlessly: "But your antidote is a poison too – of course it is, disrupting ion flux one way is as bad as disrupting it another way – it's also slow acting, though, so that you can last plenty of time after taking it. Now, there's only a couple of types of slow acting poisons to counteract Batrachotoxin that you can get your hands on, and judging by the severity of your tremor, breathing problems, perspiration, and slurring when you talk, you've taken … tetrodotoxin: essence of puffer fish! That or you've overdosed on cocaine, and I rather think I can recognize the more subtle symptom differences."

Sherlock's smirk widened slowly into a predatory smile as he took a step forward. The cabbie stumbled back from him.

"You were right," Sherlock breathed, "I _was_ stalling, not to delay _my_ taking of the pill, but to delay _yours_… Because unlike me, you _need_ to take this pill a within given amount of time after you took the antidote, or you will die from the tetrodotoxin poisoning. You were right to check your watch, because by the looks of things,_ your time has run out._"

The cabbie maintained his frozen expression for a moment longer. "Well …" he muttered, as if dazed, and then let out a string of low, resigned chuckles.

"Well, you got it all, Mr. Holmes, fair and square … I suppose there's no fooling you anymore," The cabbie popped his white capsule into his mouth, chewed, and swallowed. "Now I got nothing over you, and you can walk away, and so long as you don't attack me, I won't be using this knife … seeing as we'll both live, guess we'll call it a tie, Mr. Holmes." The cabbie lurched towards the door, pocketing the knife with a now obviously trembling hand.

One step of his long legs placed Sherlock between the cabbie and the door. "I said you made _four _major mistakes and I'm not finished with them yet," Sherlock said coldly. "Do you really think I would just let you go?" Ignoring the cabbie's livid expression, Sherlock continued in a low, threatening tone, "Your greatest mistake was to have forgotten about your aneurysm. A blood bubble in you brain just waiting to pop … you were no doubt prescribed antihypertensive drugs – lower the blood pressure, prolong your life – and you would have taken them, you want to live to make more money to leave for your children … Unfortunately for you, three years of taking antihypertensive drugs full of ion channel blockers would make you that much more susceptible to the effects of tetrodotoxin, and you didn't take that into account, did you?"

The cabbie staggered back, heaving his chest, his eyes clouding with confusion and for the first time, a hint of fear. Sherlock stalked the cabbie slowly like a cat would a mouse, relentless: "You did your research, obviously, and planned everything else meticulously – factored in your weight and the amount of tetrodotoxin you took, and gave yourself – forty minutes? forty-five? – before you have to take the pill – carried out your kidnappings efficiently, then threatened your victims when the time got near your deadline. But you've always paid more attention to your watch than to your symptoms and you never noticed that they've been worse than they should be. So far, for four times, you've been lucky. _Maybe God just loves you_. But not this time, because I've been stalling – did I mention? Because of antihypertensive medication, you always had only about thirty minutes or so before tetrodotoxin started killing you and today, you had to have taken it in the five minute time window between when we chased down your cab and when you showed up on my door step, which was what, forty-five minutes ago, by your own watch?"

Sherlock's grin was the smile of the fox that had cornered the rabbit. His low voice was triumphant as he continued: "You've waited too long, and now the tetrodotoxin has taken its toll – look at yourself, your body coordination has been deteriorating even after taking the pill, your breathing is clearly obstructed, your tremors have gotten worse!"

The cabbie raised his quivering hands and stared at them as if he had never seen them before, his chest heaving and rasping.

Sherlock watched the cabbie's dawning comprehension with cold satisfaction in his grey eyes. "You had less time than you thought you had, and when I said time had run out, I didn't mean it was running out, I meant it had already _run out_! That pill, taken too late, was not enough to counteract the tetrodotoxin, and I would say 'I guess', but I never guess: I _know_ that the only thing that could possibly save your life now is the only extra bit of Batrachotoxin you have left, and look who has it." Sherlock dangled his white capsule in front of the cabbie's face, his expression quite a lot more than a little bit smug. "It's not a tie, my good cabbie, I _won_. Now, what do you say we go to the nearest hospital where they'll inject a dilute solution of this pill into you with a nice IV, and I can have you arrested from there?"

The cabbie stared half-crazed at the capsule pinched in Sherlock's fingers for a moment, then raised his knife with a violently shaking hand, a hoarse gurgling noise rising from his throat. Sherlock lifted his eyebrows skeptically, and opened his mouth again to speak –

BAM – BANG –

Two loud explosions rang through the empty classroom in quick succession, the first one as John Watson burst through the door, the next as he took in the scene before him in a split second and fired a shot at the cabbie brandishing a knife at Sherlock. The cabbie uttered a strangled cry and dropped the knife, clutching one shoulder as dark blood spilled from it.

Sherlock recovered quickly, bending over the wounded cabbie, now spread-eagle on the floor, gasping and bleeding. "Perhaps forgetting your aneurysm wasn't the biggest mistake you made after all," He murmured, his eyes narrowed thoughtfully, "Your fourth and greatest mistake was to let your latest victim plant her cell phone on you." Sherlock crouched, bringing himself nose to nose with the cabbie: "She was cleverer than you, and now the GPS on her cell phone has lead John and the police straight to you, and not even a real gun would help you escape now, is that not so?"

The cabbie's eyes had become inhuman with anger, his face twisted, sweating, demented. His voice, barely functional, was rasping something: "You – wait… he'll – get you …_Mor'aty_."

Sherlock leaned forward further, pure interest glimmering in his eyes. In a sudden, wild swipe, the cabbie reached up and snatched the white capsule from Sherlock's hand and, in a surprisingly swift movement, put it in his mouth and swallowed it whole.

"Don't," Sherlock waved back John, who had raised his gun at the cabbie's sudden movement, and now, seeing what the cabbie had done, had rushed forward to help him. "You can't save him," said Sherlock, his face expressionless as the cabbie writhed on the floor in his own blood, eyes wide and staring horribly, convulsing with the effects of poison dart frog toxin like his four victims did before him.

* * *

><p>"No, it was obvious," Sherlock snapped, his impatience with DI Lestrade's clearly inferior intelligence not the least bit subtly hidden. "He looked focused at the moment when he was handing me the bottle but not just before, which was when he should have been deliberating which one to push forward – not to mention his breathing quickened when I seemed about to swallow the pill but not when I was choosing a bottle – conclusion: he was anticipating my taking the pill but didn't care which one I chose. There are a few other less obvious signs, but these two alone indicated clearly that he had poisoned both pills. Child's play."<p>

Sherlock paused to take a breath before continuing to answer Lestrades's questions while ignoring Lestrade's requests for him to slow down: "It _was_ suicide – he had to weigh the poisons to the milligram to calculate how much time he had, so of course he knew a full second dose would kill him; plus I already told him he could only be saved by taking a controlled partial dose. If you don't believe me, check his corpse! Even Anderson will be able to tell you he didn't die of the gun shot – and I don't need this blanket, I'm not in shock!" Sherlock tossed the offending orange blanket on to Lestrade's witness' statement notes and stalked off to find John, who was still being questioned by Sgt. Donavan.

* * *

><p>Hours later at Baker Street, after John had gotten over his lingering excitement and relief, he sat in his arm chair, broodingly quiet, for long enough that he got on the nerves of even his self-proclaimed sociopathic flat mate.<p>

Sherlock sighed in a way that was partially a groan, sat up on the couch from the lazy position on his back he had been presently sprawled in, and started to talk.

"It was the logical thing to do, from his perspective." Sherlock began without preamble, causing John to jolt out of his most recent reverie, startled. "If he were arrested alive, then the first thing this Moriarty would do would be to keep him quiet by threatening his children. Then he would probably be assassinated. That's even assuming Moriarty does not find out that he had given me the name already. By committing suicide, he's sealing his own lips, assuring Moriarty that his secrets will be kept. On the whole, his own death was unimportant to him when taking into account the facts that he was dying in any case, he most certainly would have gotten the severest sentence for the serial killings had he lived, and his children will be safe so long as I don't publically proclaim that I got the name 'Moriarty' from him, which I won't, primarily for reasons of strategic advantage. Considering his sins and his priorities, he ought to be quite satisfied."

John blinked.

Sherlock gave another sigh. "Or, if you like, you couldn't have saved him, he was dying anyway, he probably deserved it, and he would have wanted his kids safe. And yes, I had already thought of all this when I stopped you from helping him."

"Ah." said John, and though he did not make further, more eloquent comments on Sherlock's impromptu speech, John's brow cleared considerably. "I'm sorry –"

"It was hardly your fault."

"I _regret_ that I couldn't get there to shoot him earlier. Then you might not have been in any of the damned business with the poisoned pills at all."

Sherlock blinked. "Ah, well, I would've _regretted_ missing such a delightful problem," he replied as he dropped back on to the couch. "And anyway, I knew you would be showing up soon. I knew exactly how it was going to end. I was bidding my time."

"Right. Just like you knew he was using poison from…frogs and puffer fish, I think you told Lestrade?"

Sherlock's roll of the eyes was almost audible. It was a mark of the good mood Sherlock was in that he started explaining his deductions to John without once insulting John's intelligence, and that he followed his lecture about steroidal alkaloid poisons with out-loud speculations of who or possibly what Moriarty might be in a tone that was positively gleeful.

END

AN: Sincerest thanks for reading! Brutal but constructive criticism welcome and very much needed. All medical facts completely fiction, except for maybe the spelling. Sorry, medical people, for the heresy.


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